8gb limit in NT

I have an NT server running Exchange 5.5 sp3. I cloned the original 8gb disk to a 20 g drive, and installed a second 20g drive to be used as a mirror set. Both harddrives only being seen as 8gb, even with service pack 6A installed. The computer is a Dell xps 350 pentium II. The first drive shows the full size of both partitions, but is only reported as 8gb total capacity.